1. Field of the Invention
This invention relates to a method and apparatus for fluidized catalytic cracking. More particularly, it relates to improvements in the feeding of hydrocarbonaceous materials to a cracking apparatus by split injection, whereby at least a portion of the feed is injected into a downstream section of a riser reactor pipe, the downstream section being contained within a separation chamber. The split feeding enhances gasoline octanes and product yields.
2. Description of the Prior Art
Fluidized cracking of heavy petroleum fractions is one of the major refining methods used to convert crude petroleum oil to useful products, such as fuels for internal combustion engines. In such fluidized catalytic cracking (known popularly as "FCC"), high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided solid catalyst particles in an elongated riser or transfer line reactor. The riser reactor is usually in the form of a riser pipe and the contacting time is on the order of a few seconds, say from 1 to 10 seconds, and generally not over about 5 seconds. During this short period, catalysts at temperatures in the range of from about 1100.degree. F. to 1400.degree. F. are contacted with a hydrocarbon feedstock, frequently in the form of vacuum gas oil, cycle oil or the like, heated to a temperature of from about 300.degree. to 800.degree. F.
The reaction is one of rapid generation of large volumes of gaseous hydrocarbons. The hydrocarbons and catalyst mixture flow out of the riser reactor into a separator vessel wherein the resultant gaseous hydrocarbons are taken off for distillation into various product fractions defined by boiling ranges. The spent catalyst is separated in the separator vessel as rapidly as possible and stripped of residual hydrocarbons by passage through a stripper section. The catalyst is then returned to a regenerator where residual hydrocarbons on the spent catalyst, called "coke," are burned off by passing a stream of an oxygen-containing gas, such as air, or oxygen enriched air, through them. The heat generated in this regeneration step is used as the heat source which heats the catalyst and thus provides the elevated temperatures needed for reaction with the incoming hydrocarbon feed. Regenerated hot catalyst is then recycled by gravity primarily for further reaction with feed in the riser reactor.
There are generally two arrangements for conveying reacted hydrocarbons and spent catalyst from the riser reactor to the separator vessel. The first is through an "external" riser reactor in which the end section of the riser reactor passes through the wall of the separator vessel. Substantially all of the reaction in the riser reactor takes place prior to the mixture reaching the separator vessel. The end section enters through the sidewall in the upper portion of the vessel where the majority of separation occurs.
The other arrangement is for a portion of the riser reactor, including the end section, to be located within the separator vessel, i.e., an "internal" riser. The riser reactor enters the separator vessel at the lower portion thereof and extends substantially vertically into the upper portion. A substantial amount of the reaction takes place in the internal section located within the separator vessel.